The market for modern switching power converters is demanding higher power levels and power densities. Meeting this demand requires that components become smaller and dissipate less power. Component size reductions generally require an increase in switching frequency while improving efficiency. At the same time electromagnetic interference (EMI) generated by the higher frequency switching of voltages and currents must be at or below prior art levels to meet federal and international requirements.
One approach to meeting these conflicting requirements is the use of various "soft switching" power converters. In one type of power converter, the voltage is brought to zero on the main switch or switches prior to turning on. When the switch is turned off, the current transfers to the junction capacity of the switch or switches or to additional capacity placed across the switch or switches to assist in reducing EMI.
In our co-pending application Ser. No. 07/909,257 filed Jul. 6, 1992, is disclosed soft switching circuits for buck and boost regulators which utilize tapped "main" inductors. In one embodiment, a small "pilot" inductor is disclosed in series with the tap of the main inductor. The use of discrete pilot and main inductors is known with the pilot inductor consisting of a single winding on a core while the main inductors consists of either a tapped winding on the core or possibly a voltage bucking winding in addition to an untapped winding to create the effect of a tap on the main winding.
The pilot inductance in our previously filed application is small compared to the main inductor and the RMS current is considerably less in the pilot inductor, although peak currents are similar. Thus, the pilot inductor is electrically smaller than the main inductor, but in practice the dimensions of the pilot inductor are comparable to those of the main inductor.
This discrepancy in relative sizes between the main inductor and the pilot inductor is at least partially due to the fact that while the main inductor current and flux are largely DC with a smaller superimposed AC component, the winding current and core flux in the pilot inductor pulse from zero to maximum and back to zero very quickly. The pulse duration is typically in the order of 5% to 10% of the switching period, which generates strong harmonics of ten or twenty times the switching frequency. Winding and core losses increase dramatically with frequency above present switching frequencies of 50 to 200 KHz, resulting in winding and core losses in the order of three to ten time higher than would be expected from the RMS current and the peak-to-peak core flux at the switching frequency.